Oceans will lose ability to absorb heat
By Summit Voice
FRISCO — Keeping global warming below 2 degrees Celsius may require a more dramatic turn away from fossil fuels than previously believed, researchers said last week, describing a greenhouse gas lag that could cause temperatures to keep rising even after CO2 emissions stop.
The Princeton-led study, published in the journal Nature Climate Change, suggests that it might take a lot less carbon than previously thought to reach the global temperature increase scientists deem unsafe. Temperatures would initially drop after CO2 levels stabilize, but eventually, the world’s oceans would lose some their capacity to take up heat, especially in polar regions, the study found.
In their study, the scientists modeled a scenario that halted all CO2 emissions after 1.8 billion tons carbon entered the atmosphere, a simulation often used to measure the staying power of heat-trapping gases. The model shows that oceans and forests would absorb about 40 percent of the CO2 within 40 years and 80 percent after 1,000 years.
That should lead to cooling, but the model showed that the trapped by the carbon dioxide took a divergent track. After a century of cooling, the planet warmed by 0.37 degrees Celsius (0.66 Fahrenheit) during the next 400 years as the ocean absorbed less and less heat. While the resulting temperature spike seems slight, a little heat goes a long way. Earth has warmed by only 0.85 degrees Celsius (1.5 degrees Fahrenheit) since pre-industrial times.
According to current estimates, staying below the 2 degree Celsius threshold would require keeping CO2 emissions below 1,000 billion tons of carbon, about half of which has already been put into the atmosphere in the past 150 years, but the lingering warming effect the researchers found suggests that the 2-degree point may be reached with much less carbon.
“If our results are correct, the total carbon emissions required to stay below 2 degrees of warming would have to be three-quarters of previous estimates, only 750 billion tons instead of 1,000 billion tons of carbon,” said Thomas Frölicher, a researcher at the Swiss Federal Institute of Technology in Zurich. “Thus, limiting the warming to 2 degrees would require keeping future cumulative carbon emissions below 250 billion tons, only half of the already emitted amount of 500 billion tons.”
The key finding identified by the study is that there will be a gradual decline in the oceans’ ability to absorb heat from the atmosphere, particularly the polar oceans, Frölicher said. Although carbon dioxide steadily dissipates, Frölicher and his co-authors were able to see that the oceans that remove heat from the atmosphere gradually take up less. Eventually, the residual heat offsets the cooling that occurred due to dwindling amounts of carbon dioxide.
Frölicher and his co-authors showed that the change in ocean heat uptake in the polar regions has a larger effect on global mean temperature than a change in low-latitude oceans, a mechanism known as “ocean-heat uptake efficacy.”
“The regional uptake of heat plays a central role. Previous models have not really represented that very well,” Frölicher said.
“Scientists have thought that the temperature stays constant or declines once emissions stop, but now we show that the possibility of a temperature increase can not be excluded,” Frölicher said. “This is illustrative of how difficult it may be to reverse climate change — we stop the emissions, but still get an increase in the global mean temperature.”
The paper, “Continued global warming after CO2 emissions stoppage,” was published Nov. 24 by Nature Climate Change. Funding for the work was provided by the Swiss National Science Foundation (Ambizione grant PZ00P2_142573) and Princeton University Carbon Mitigation Initiative.